Much work has been done, particularly in recent years, regarding apparatus for combining informatin from multiple sources for display on a common output device, such as a television. These efforts have, for example, included apparatus for adding textual, data or graphics display to a televised video signal.
Exciting possibilities have been suggested with the advent of a new recording medium, the video disc, and a source of video signals, the video disc player. The video disc is a rotating medium which typically can store up to 54,000 frames of addressable video images in standard television (e.g., NTSC) format, with accompanying audio. These can be displayed as up to 30 minutes (or more) of moving sequences, or as individual still frames, with no restriction on the time duration of the still frame mode. The video disc player, the machine which reads information stored on a video disc, is a random access device in which each frame may be called up for display within an average seek time of about 3 seconds. Due to this ability to switch rapidly from one video frame to another on the disc, video discs are a good medium for storing records, such as inventory files which must be consulted frequently, and for storing the video portion of so-called courseware for computer-based instruction (i.e., the material to be presented to the student). Rapid switching of frames and frame sequences is important in order for the instructional sequence to be responsive to input from the student. That is, if a student gives a correct response to a question, the course must advance to a first preselected frame; but if he or she gives an incorrect response, it must advance to a second, different, preselected frame. Indeed, with this capability, it may also be possible to use the same recorded video information for different courses by presenting it in different sequences.
Clearly, the scenario just discussed is one which assumes the interaction of a video disc player with a computer which evaluates student responses and causes the video disc player to choose its display sequence in accordance therewith. A commercial video disc player such as used herein includes a computer interface through which it can be controlled by the courseware program running in an external processor, and external synchronization inputs through which it can be somewhat, but not completely, synchronized to the remainder of the video system. The above-referenced commonly-assigned application titled Interactive Computer-Based Information Display System relates to such a use of the apparatus described herein.
One of the most significant problems in mating a video disc player with a computer for providing computer-based instruction or image retrieval with graphics/text overlay as outlined herein is that of synchronizing the video output from the computer with the output from the video disc player, since very precise placement of both images is needed. With a high resolution display which normally is viewed at close distances, such as a video display terminal which would be used for educational purposes, the synchronization error and jitter must be significantly less than the size of one pixel (picture element) or phosphor dot on the display; otherwise, the graphics or textual display will not line up vertically from one line to the next; as a result, the user will find the display jittery, uncomfortable and fatiguing to watch and unsatisfactory for use. The situation is particularly egregious when the video source is a video disc player (VDP), since the VDP is a rotational mechanical device lacking precise time base correction. It therefore exhibits a large amount of horizontal jitter. This jitter usually takes the form of large jumps in the temporal position of the output composite video signal, including the horizontal sync pulse thereof, relative to the "house" sync input to the player or the player's internal sync source. The magnitude of this jitter frequently is as wide as one or two complete characters on the display, which obviously is unacceptable. Expensive laboratory-type equipment exists for supplying a time-base correction to the video disc player's output in order to provide a stable display. This equipment, though, is so expensive as to be absolutely useless in a commercial product of the type envisioned herein.
Combining the video disc output with computer-generated text or graphics output leads to other substantial problems, also. In the prior art, the approach generally has been to convert the computer video signals to NTSC (or other compatible) composite video signals and then to produce the combined display by switching between that signal and the NTSC signal from the video disc player, such as switching with convential "chroma key" switching. Because the phase of an NTSC composite video signal contains the encoded color information, and phase cannot be matched perfectly when switching, this approach sacrifices color purity. And encoding any video signal, especially a high resolution signal, in the NTSC format sacrifices resolution and introduces dot crawl, rainbows and smearing due to bandwidth restrictions. Moreover, because of the manner in which the NTSC signal is recorded on the video disc and the techniques used to do still frame display, the color subcarrier phase is shifted on a frame-to-frame basis. If the graphics/text source is to be encoded into and merged as an NTSC signal, severe color shifts may result. The only cure known to date is to use an indirect color-time base corrector or frame buffer which decodes, stores and reencodes the NTSC signal. Its cost, unfortunately, is quite large. For this reason, NTSC overlay of a video disc signal is technically impractical outside the laboratory or sophiscated television studio.